Difunctional aldimines and ketimines, that is condensates of diamines with aldehydes or ketones, or, the condensates of dialdehydes or diketones with primary amines, are known in the prior art. The use of aldimines and ketimines in urethane coatings and sealant compositions is also known. For example, their use as-catalysts for urethane coatings and sealant systems are described in U.S. Pat. Nos. 3,553,118 and 3,637,903. Other disclosures of aldimines and ketimines in the prior art are largely directed at their use in moisture cure systems: for example, together with blocked isocyanate terminated. prepolymers (see U.S. Pat. Nos. 3,267,078; 4,507,443; and 4,847,319); and, with free isocyanate curatives (see U.S. Pat. Nos. 3,420,800; 3,440,087; 3,567,692; 3,589,932; 3,629,013; and 3,793,417). This last patent, U.S. Pat. No. 3,793,417, shows the use of systems based on aldimines, isocyanates and polyols in moisture and in heat cured coatings.
The bis(imine) reaction product of 1,4-diaminobutane with isobutyraldehyde is known and disclosed, for example, in U.S. Pat. No. 2,416,042. Additional information relating to the production and photochemistry of these compounds is disclosed in a 1985 technical journal article (J. Inst. Org. Khim., Moscow, USSR, 1985, 7, 1612-15) and in a 1991 technical journal article (Bittner and Rademacher, J. Chem. Research (S), 1991, 60-61), respectively.
Japanese Kokai JP 07 18,051, published on Jan. 20, 1995, discloses a polyurethane elastomer-forming composition containing, inter alia, a urethane prepolymer of 2,4-TDI, polyoxypropylene glycol, polyoxypropylene triol and a dialdimine obtained by reacting 1,4-diaminobutane and p-tolualdehyde. This elastomer-forming composition is said to exhibit good storage stability.
U.S. Pat. No. 5,214,086 discloses a plethora of aldimine and ketimine compounds within an empirical structural formula shown for such compounds. The compounds of the '086 patent are disclosed therein as reactive diluents for reducing the volatile organic content ("VOC") of a paint composition by increasing the solids content of the paint without adversely affecting the paint's performance.
Reactive diluents, such as the above-discussed difunctional aldimines are referred to as diluents since they have low viscosities, and are reactive, inasmuch as they participate in the film forming reaction during the formation of a coating. In view of these characteristics of reactive diluents, they are suitably employed to replace some or most of the solvent that is otherwise needed to reduce the viscosity of a coating formulation to a desired low viscosity, thereby reducing the quantity of VOCs emitted by the system, an advantage from an environmental and toxicology standpoint.
It has now been found by the present inventors that previously known aldimine and ketimine reactive diluents, such as those specifically described, for example, in the above-discussed '086 patent, suffer from a key deficiency. More specifically, these reactive diluents have relatively low solubility in polyisocyanates that comprise HDI isocyanurates ("trimer") and/or HDI biurets. Since a completely miscible system is a prerequisite for insuring a homogeneous film to provide good physical properties in the resulting coating, such low solubility reactive diluents require the use of additional formulation steps and/or solubilizing additives to permit the use of these reactive diluents in coating compositions to provide high performance coatings having good physical properties. Illustrative steps and solubilizing additives needed when using these prior art reactive diluents include, for example, the following: (a) using higher levels of solvents, which undesirably increases the formulation's VOC content; (b) replacing some or most of the isocyanurate or biuret polyisocyanate with an HDI uretidione based polyisocyanate, which reduces the average isocyanate functionality of the system and thereby reduces the crosslink density and the corresponding physical properties of the resulting film; and/or, (c) using lower molecular weight, lower functionality isocyanate reactive resins, which, likewise, reduces the crosslink density and performance characteristics of the resulting film. The second (b) and third option (c) also restrict the number of suitable isocyanate curatives and isocyanate reactive resins available to the coatings formulator, thus rendering the development of such systems more inflexible than otherwise might be desired.
Thus, there is a heretofore unsatisfied need in the marketplace for an imine-type reactive diluent that is completely soluble in HDI isocyanurate and biuret based polyisocyanates. Several such highly desirable reactive diluents are provided by virtue of the present invention.